Dislocation Density Distribution Research Articles

A study of unstable fracture of a magnesium alloy caused by uneven microstructure

The influence of the uneven microstructure of the magnesium alloy sheet on the slip and twinning behavior during the room temperature uniaxial tension was investigated using slip trace analysis and EBSD-based misorientation analysis, and the reasons for the instability and fracture of the uneven microstructure are easily explored. The results show that the uneven microstructure makes the distribution of dislocation density and strain uneven, the stress is concentrated in small grains, and the intergranular fracture occurs at the boundary between the large and small grains. Large misorientation may cause slip to fail to be transferred between grains, and dislocations accumulate at the grain boundaries, leading to instability and fracture.

Application of the Alexander–Haasen Model for Thermally Stimulated Dislocation Generation in FZ Silicon Crystals

Numerical simulations of the transient temperature field and dislocation density distribution for a recently published silicon crystal heating experiment were carried out. Low- and high-frequency modelling approaches for heat induction were introduced and shown to yield similar results. The calculated temperature field was in very good agreement with the experiment. To better explain the experimentally observed dislocation distribution, the Alexander–Haasen model was extended with a critical stress threshold below which no dislocation multiplication occurs. The results are compared with the experiment, and some remaining shortcomings in the model are discussed.

Numerical Analysis of Dislocation Density of SiC Crystals Tilted from [0001] Toward [12¯10]$[ {1\bar 210} ]$ and [11¯00]$[ {1\bar 100} ]$ Grown by Physical Vapor Transport

AbstractA 3D and time‐dependent analysis of dislocation density using the Alexander–Haasen model to estimate the plastic deformation of tilted angle growth of 4H–SiC single crystals during crystal growth and cooling processes grown using the physical vapor transport method is performed. The growth direction is set to tilt from [0001] toward and to fabricate wafers to stabilize polytype during crystal growth of the epitaxial layer. The maximum dislocation density in SiC crystals increases as the tilted angle increases, whereas von Mises stress, which is one of the measures of dislocation density, decreases. The calculated results show dislocation density distribution in the crystal grown in [0001] exhibits six‐folded symmetry, whereas that in the crystal grown in the tilt direction from [0001] toward and exhibits asymmetric distribution.

Laser Shock Peening Induced Back Stress Mitigation in Rolled Stainless Steel

Abstract Laser shock peening (LSP) is investigated as a potential tool for reducing tensile back stress, shown here applied to rolled and annealed 304L austenitic steel. The back stress of treated and untreated dog-bone samples is extracted from hysteresis tensile testing. Electron back-scatter diffraction (EBSD) and orientation imaging microscopy (OIM) analysis quantify the geometrically necessary dislocation (GND) density distribution of unstrained and strained as well as unpeened and peened conditions. Finite element analysis (FEA) simulation models back stress and residual stress development through tensile testing and LSP treatment using known LSP pressure models and Ziegler's nonlinear kinematic hardening law. Nonlinear regression fitting of tensile testing stress–strain in as-received specimens extracts the kinematic hardening parameters that are used in numerical study. This research shows LSP may be used to overcome manufacturing design challenges presented by yield asymmetry due to back stress in rolled steel.

Dislocation density in cellular rapid solidification using phase field modeling and crystal plasticity

A coupled phase field and crystal plasticity model is established to analyze formation of dislocation structures and residual stresses during rapid solidification of additively manufactured 316L stainless steel. The work focuses on investigating the role of microsegregation related to the intra-grain cellular microstructure of 316L. Effect of solidification shrinkage is considered along with dislocation mediated plastic flow of the material during solidification. Different cellular microstructures are analyzed and the characteristics of the cell core, boundary and segregation pools are discussed with respect to heterogeneity of dislocation density distributions and residual stresses. Quantitative comparison with experimental data is given to evaluate the feasibility of the modeling approach.

Mode I and mode II stress intensity factors and dislocation density behaviour in strain gradient plasticity

In this study, we use the mechanism-based strain gradient plasticity theory to evaluate both crack tip dislocation density behaviour and the coupled effect of the material plastic properties and the intrinsic material length on non-linear amplitude factors. The two planar classical stress–strain states are examined, namely, plane strain and plane stress, both under pure mode I and pure mode II loading conditions. The constitutive relations are based on Taylor’s dislocation model, which enables gaining insights into the role of the increased dislocation density associated with large gradients in plastic strain near cracks. The material model is implemented in a commercial finite element (FE) software package using a user subroutine, and the nonlinear stress intensity factors (SIF) are evaluated as a function of the intrinsic material length, characterising the scale at which gradient effects become significant. As a result of the FE calculations of dislocation density distributions, the effects of both the fracture mode and the stress–strain state are determined. In pure mode I, the geometrically necessary dislocation (GND) density is located symmetrically with respect to the blunted crack tip. On the contrary, under pure mode II, the GND density becomes concentrated in the blunted and sharp parts of the crack tip. In this case, fracture initiation is shown to be likely to occur near the blunted region of the crack tip, where both the stress triaxiality and the GND density are at their maximum. The relation between the equilibrium state of dislocation densities and the intrinsic material length as well as the plastic SIF as a function of the work hardening exponent is discussed.

The microstructure evolution history and the underlying mechanism of an α/β dual phase titanium alloy during elastic-plastic deformation process

In this paper, in situ scanning electron microscope tensile test was carried out on the Ti-4.5Mo-5.1Al-1.8Zr-1.1Sn-2.5Cr-2.9Zn alloy, and the mechanism of grain rotation, shape change and micro-strain redistribution in macroscopic elastic deformation (stage I), macroscopic elastic-plastic transition (stage II) and macroscopic plastic deformation (stage III) stages were quantitatively investigated by electron backscatter diffraction and digital image correlation. In stage I, it was found that 10.1% of the viewing zone underwent grain rotation or shape change, and the micro-strain was mainly distributed in primary α phase (αp) and their intersection. The result of the separately designed elastic loading-unloading tensile test showed that the rotated αp no longer returned to the primary orientation. In stage II, the tensile plastic deformation regions reached 52.9%, and grains in βT regions continued to rotate and began to change shape. In stage III, 84.6% of the viewing zone got into the tensile plastic deformation state. Combined with geometrically necessary dislocation density distribution, it was found that some of the regions in low strain state (0 ≤ ε<1%) actually underwent complex tensile plastic deformation. In addition, the maximum micro-strain of the viewing zone was transferred to βT regions.

Analytical Evaluation of Strengthening, Local Internal Stresses and Microderformations in Welded Joint Metal during Wet Underwater Welding

Analysis of structural factor influence on local internal stresses and zones of deformation localization in upper and lower bainite structures in welded joints of low-alloy steel at wet underwater welding was performed. It is established that when welding joints under the water and applying an external electromagnetic field in the metal of the heat-affected zone (HAZ), a finer-grained substructure is formed with a general decrease in the dislocations density and with their uniform distribution. Estimates of the local internal stresses level considering the dislocation density distribution in the structural zones of their localization show that their maximum level is formed in the metal of the HAZ overheating region at welding without the external electromagnetic field along the upper bainite laths boundaries. The upper bainite structure is characterized by forming localized deformation zones, where the most significant dislocation density gradients are observed. This can lower the crack resistance of welded joints. Low values of local internal stresses are characteristic of welded joints obtained in the modes applying an external electromagnetic field. This is facilitated by the overall decrease in the dislocation density and their uniform distribution in the lower bainite structural components, which provides high crack resistance of welded joints.

Analyzing recrystallization behavior of heterogeneous structures single-phase Al alloys

In this research, mixed structures are created so that they have regions where dislocations accumulate and a region that is, to some extent, free from dislocations. These heterogeneous structures that bring together both soft and hard domains in a single sample are presumed to produce a fine recrystallized grain and recovered large grain structures if they are subsequently heated, leading to limited recrystallization. In the present work, the 3D Potts model has been modified to tailor and model complex problems such as heterogeneous structure recrystallization. This is done by introducing a limited fraction of nucleating grains to the structure where the highest geometrically necessary dislocation density distributions develop in the rolled layers at early simulation time as well as by controlling structure evolution in the undeformed structure layer using grain boundary energy criteria. Additionally, the designed microstructures were investigated using electron backscatter diffraction measurements and an in-situ heating stage performed on material with various degrees of deformation.

Shape controlling and property optimization of TA32 titanium alloy thin-walled part prepared by hot forming

The hot flow behaviors, microstructure evolution and fractographs were studied to optimize the hot forming process of the TA32 titanium alloy thin-walled part. A set of microstructure-based constitutive equations were developed based on the experimental data, which described the relationships among the hot flow stresses and the evolution of phase volume fraction, dislocation density, grain size and damage. The constitutive model was imported into ABAQUS 6.14 to simulate the hot forming process for a typical thin-walled part. The effective strain, dislocation density and damage distribution as well as forming defects of formed parts under different process parameters were predicted. A qualified part without wrinkling and fracture defects was produced at a loading speed of 5 mm/s at 800 °C by the modified blank shape, where the maximum damage value was only 18.3%. The accuracy of constitutive model and finite element (FE) simulation was verified by the microhardness tests, which indicates that the FE model based on physical internal-state variables can well optimize the hot forming process of TA32 titanium alloy complex parts.

A novel multiscale material plasticity simulation model for high-performance cutting AISI 4140 steel

The achievable machined surface quality relies significantly on the material behavior during the high-performance cutting process. In this paper, a multiscale material plasticity simulation framework is developed to predict the deformation behaviors of AISI 4140 steel under various high-performance cutting conditions. The framework was built by coupling a three-dimensional discrete dislocation dynamic (3D-DDD) model with a finite element method (FEM) through the optimization of a dislocation density-based (DDB) constitutive equation (compiled as a user-defined subroutine in ABAQUS). The movement of edge and screw dislocations such as generation, propagation, siding, and their interactions, was performed by 3D-DDD, and the statistical features of dislocations were used to optimize the critical constants of the DDB constitutive equation. For validation, a classic FEM cutting model (Johnson-Cook constitutive equation) was employed as a reference. The simulation results indicated that the proposed multiscale model not only can precisely predict the stress, strain, cutting force, and temperature as those predicted by the classic FEM simulations, but also capture the microstructure characteristics such as grain size and dislocation density distributions under the tested cutting conditions. Severe dynamic recrystallization phenomena were found at the core shear zones. The recrystallization process reached a dynamic equilibrium at the machined surfaces when the cutting speed is larger than 280 m/min or the external-assisted temperature is between 200 and 350°, indicating an optimal range of machining parameters for improved surface integrity.

Origins of the mechanical property heterogeneity in a hybrid additive manufactured Hastelloy X

Hybrid additive manufacturing (hybrid-AM) technique is a new processing technique in which the large part of a component is fabricated by conventional manufacturing techniques and the complex parts are added flexibly via AM. This technique provides an efficient and economical method for manufacturing metallic components with different degrees of complexity. However, the mechanical properties of the substrate and the additive manufactured zone usually failed to match well while the underlying mechanism is still not fully understood. To address this problem, here we revealed the mechanisms of mechanical property heterogeneity in a hybrid-AMed Hastelloy X by microstructural characterization, thermo-mechanical simulations, and strengthening mechanism analyzing. The results demonstrate that the tensile properties and micro-hardness along the building direction show distinctive heterogeneity in the hybrid-AMed Hastelloy X, which is directly related to the evident microstructure changes. We attributed the microstructural variation to the different thermal histories of different regions during AM. Strengthening mechanism analysis indicated that the inhomogeneity of mechanical properties in the hybrid-AM Hastelloy X was mainly determined by the heterogeneous distribution of grain size, dislocation density, and crystallographic texture. EBSD, ECCI, and residual stress simulation all provided strong evidence for this conclusion. These findings shed new insights not only for achieving more homogeneous mechanical properties during the hybrid-AM process but also for laser additive repair and other fields.

Analyzing Recrystallization Behavior of Heterogeneous Structures Single-Phase Al Alloys

In this research, mixed structures are created so that they have regions where dislocations accumulate and a region that is, to some extent, free from dislocations. These heterogeneous structures that bring together both soft and hard domains in a single sample are presumed to produce a fine recrystallized grain and recovered large grain structures if they are subsequently heated, leading to limited recrystallization. In the present work, the 3D Potts model has been modified to tailor and model complex problems such as heterogeneous structure recrystallization. This is done by introducing a limited fraction of nucleating grains to the structure where the highest geometrically necessary dislocation density distributions develop in the rolled layers at early simulation time as well as by controlling structure evolution in the undeformed structure layer using grain boundary energy criteria. Additionally, the designed microstructures were investigated using electron backscatter diffraction measurements and an in-situ heating stage performed on material with various degrees of deformation.

Experimental and analytical investigation on service life of film cooling structure for single crystal turbine blade

In this study, regarding the film cooling structure of single crystal (SC) turbine blade, the life tests simulating the service condition are conducted, and the life prediction modeling as well as damage mechanism analysis are carried out. In life test, the SC specimen with multiple film cooling holes is designed to simulate the geometrical characteristic of film cooling structure. The creep-fatigue loading spectrum characterizing a typical flight process is employed. The loading system is developed to simulate the cyclic service load in laboratory environment. Meanwhile, the strain distribution of film cooling structure is measured using digital image correlation (DIC) method. After that, the life tests are conducted, and eight groups of test lives are obtained. Taking the strain distribution measured by DIC as input, the model based on theory of critical distance (TCD) is improved to predict the service lives of film cooling structure. Furthermore, the crack nucleation mechanism of film cooling structure is discussed. Based on the microscopic observation, it can be found that the microcrack nucleation around hole edge is induced by persistent slip bands (PSBs). The matrix channel that significantly influences the dislocation accumulation rate and PSBs formation is chosen as the concerned microstructure. Under the effect of stress distribution of film cooling structure, the uneven spatial distributions of mean values and standard derivations for matrix channel width are quantificationally revealed. And the distribution of dislocation density on film cooling structure is measured using electron back-scatter diffraction (EBSD). It can be found that the concentration of dislocation density exists around the hole edge, and the distribution of dislocation density is quite unsmooth due to the scatter of matrix channel width. These findings can be employed in the life prediction and fault diagnosis for film cooling structure under service condition.

Effects of high entropy and twin boundary on the nanoindentation of CoCrNiFeMn high-entropy alloy: A molecular dynamics study

To study the effects of twin boundary and high-entropy on elastic–plastic behavior of high-entropy alloys (HEAs), molecular dynamics (MD) was employed to simulate the nanoindentation on single-crystal CoCrNiFeMn HEA (sc-HEA), twinned CoCrNiFeMn HEA (tw-HEA) bicrystal and twinned Ni (tw-Ni) bicrystal. The deformation behaviors of the three samples were then compared with each other. Simulations revealed that the load-drop phenomenon during the indentation in the HEAs is not so apparent as that in the Ni. Microstructure characterization showed that a dense dislocation network was localized below the indentation pit of the HEAs. These phenomena are related to the damping spreading behavior of dislocations underneath the indenter. Through the analysis of the plastic zone underneath the indentation, it is found that the twin boundary inhibits dislocation penetration, and moreover, provides a slipping path for dislocations. The radial distribution of dislocation density proves that dislocations in the indentation of the HEAs are more concentrated than that in the traditional metals. Understanding the sluggish dislocation behavior and twin boundary effect help understand the deformation mechanisms underlying the mechanical response of HEAs.

Dislocation structure of the AlGaN/GaN/α-Al2O3 heterostructures epitaxial layers at doping GaN with С and Fe

The aim of this work was to study the influence of the iron and carbon doping of the epitaxial GaN layer on sapphire on the growth features of epitaxial films and their dislocation structure. Investigation methods used were: mass spectroscopy of secondary ions, selective chemical etching on spherical sections, and also single-crystal diffractometry.It is shown that doping during growth of an epitaxial GaN layer with carbon can lead to a significant decrease in the dislocation density in epitaxial layers.It has been shown that for samples doped with iron, a decrease in the number of short dislocations located in the bulk of the structure is characteristic, but a large number of extended dislocations are generated contributing to the diffusion of iron into the working regions of heterostructures, which can adverselyinfluence on the electrical parameters of the structures. In the course of the work, a technique for determining the density of dislocations in epitaxial films was proposed using two schemes of selective etching of spherical thin sections, which makes it possible to determine the dislocation density distribution over the depth of epitaxial films.

Atomistic simulations to study crack tip behaviour in multi-elemental alloys

The aim of the article is to study the crack tip behaviour in mutli-elemental alloys. Molecular dynamics based simulations were performed in conjunction with 2NN MEAM potential to study the fracture behaviour in ternary, quaternary, and quinary alloy configurations. Simulations were performed after aligning the crack plane with three different principle planes of FCC crystal. Deformation in each of the orientations was primarily governed by dislocations, twinning, and stacking faults emanating front the crack tips or from the surface of the crack plane. Medium and high entropy alloys have shown higher resistance to fracture, and were capable of retaining strength up to a larger extent as compared to low entropy alloy and pure Ni crystal. The higher dislocation density and uniform distribution of dislocation in medium and high entropy alloys leads to plasticity in the crystal after the onset of yielding. Blunting of the crack tip in medium and high entropy alloys was predicted in orientation 1 and 2, whereas crack propagates in orientation 3. This study will help in elucidating the capabilities of medium and high entropy alloys in resisting the opening mode of crack propagation, which can be further utilized to enhance the diversified structural applications of these emerging multi-elemental alloys.

分散強化合金中に蓄積する3次元GN転位密度分布の結晶塑性解析

分散強化合金中に蓄積する3次元GN転位密度分布の結晶塑性解析

Distribution of Misfit Dislocations and Elastic Mechanical Stresses in Metamorphic Buffer InAlAs Layers of Various Constructions

The equilibrium distributions of the misfit dislocation density ρ(z) and elastic stresses e(z) are calculated along the direction of the epitaxial growth of the metamorphic InAlAs/GaAs(001) layer with higher In content (to 87 mol %) and various profiles of varying the composition: step, linear, and root. The calculations are performed using the method based on iteration searching for the minimum total energy of the system. It is shown that the largest differences between various constructions of the buffer layer are observed in the character of distributions ρ(z), rather than e(z). Unlike the traditional constructions with a step and linear gradients of the composition, which are characterized by a quite homogeneous distribution of misfit dislocations, in a buffer layer with a root composition gradient, the main part of such dislocations is concentrated in the lower part of the layer near the heteroboundary with a GaAs substrate, and their density sharply decreases by more than one order of value along the layer thickness, achieving the value minimum for all abovementioned constructions. In spite of the fact that the important effect of interacting the dislocations to each other is not taken into account in this work, the calculations enable us to establish the main peculiarities of the distributions ρ(z) and e(z) in various metamorphic buffer InAlAs layers, which were observed experimentally before. Thus, this approach can be effectively used when designing optimal constructions of the device metamorphic heterostructures.

Micromechanical investigation of thermal stresses of a finite plane containing multiple elliptical inclusions based on the equivalent inclusion method and distributed dislocation method

This paper presents a micromechanical method to analyze the thermal stresses in a finite plane containing multiple elliptical inclusions. Firstly, the Eshelby’s equivalent inclusion method is employed to solve the elastic fields of a two-dimensional infinite plane containing multiple elliptical inclusions under a uniform temperature change. Both the interior Eshelby’s tensor and the exterior Eshelby’s tensor are employed. Then the boundary of the plane is modeled by continuous distributions of dislocation densities. By combining the two steps, a system of singular integral equations is formulated based on the traction-free boundary condition. Then the thermal stresses of the plane can be obtained by the superposition of the stresses obtained by the Eshelby’s equivalent inclusion method and distributed dislocation method. Additionally, some examples are given to show the effects of the presented method. The effects of the material constants, geometric parameters and fiber packing arrangement on the thermal stresses are also studied.